1. Field of the Invention
The present invention relates to a frequency generator for obtaining an optimum frequency in correspondence to the rotating speed of a d.c. motor, and to a d.c. motor equipped with such a frequency generator.
2. Description of the Prior Art
In order to explain a conventional frequency generator, reference will be made to FIG. 2, which is referenced by an embodiment of the present invention. The frequency generator is provided with a stator section S and a rotor section R. The stator section includes a driving coil 1, a stator core 2, a bearing 3, and a printed board 4 having a coil pattern 5 designed to generate a frequency. The rotor section R includes a driving magnet 6, a frequency generating magnet 7, a shaft 8, and a rotor frame 9. FIG. 5 shows a frequency generating coil pattern known in the art. The frequency generating coil pattern is formed in a zigzag pattern. A canceling coil pattern is formed inside the frequency generating coil pattern.
The frequency generator of FIG. 2 operates as follows:
The rotor section R is rotatably supported by the bearing 3 such that it can rotate with respect to the stator section S. The magnetic path of the driving magnet 6 is formed by the rotor frame 9 on one side, and is formed by the printed board 4 and the stator core 2 on the other side. When an electric current flows through the driving coil 1, electromagnetism is generated between the driving coil 1 and the driving magnet 6. The driving coil 1 is formed integrally with the stator core 2, thereby enabling the rotor section R including the driving magnet 6 to rotate as a whole. The rotation of the rotor section R changes a magnetic flux which passes through the coil pattern 5 from the frequency generating magnet 7. An output in proportion to the change in the magnetic flux appears at an output terminal (not shown) of the coil pattern 5 as a speed signal which depends upon the rotating speed of the rotor section R.
Under the prior art system described above, because of the positions of the driving magnet 6 and stator core 2 near the frequency generating coil pattern 5, the magnetic flux from the magnet 6 affects the output of the frequency generating coil pattern 5. This noise based on the magnetic flux prevents the precise rotation of the rotor section, thereby increasing wow flutter. In order to solve such wow flutter problems, the number of the polarizing poles of the magnet 7 and the pattern lines of the coil pattern 5 are increased by integral multiples of the number of polarizing poles of the driving magnet 6, and the inside diameter of the frequency generating coil pattern 5 is enlarged to the extent that it is situated as far as possible radially from the driving magnet 6. However, this method is likely to shorten the effective diameter of the generating pattern lines through which the magnetic flux of the frequency generating magnet 7 passes, thereby decreasing the output of the generating coil. And adversely affecting the signal-to-noise (S/N) ratio.